You can upgrade PHP to 5.3 without updating vB first. You just can't update to PHP 5.4.
In fact, VaultWiki 4 is not yet compatible with vBulletin 4.2.2, so you should:
1. Upgrade to PHP 5.3.
2. Upgrade VaultWiki.
3. Wait for a version compatible with vBulletin 4.2.2. Upgrade VaultWiki again at that point.
4. Upgrade vBulletin to 4.2.2.
5. Upgrade to PHP 5.5.